Apparatus for continuous and simultaneous measurement of concentration in the atmosphere of the shortlived solid decay products of radon



C. LASSEUR APPARATUS FOR CONTINUOUS AND SIMULTANEOUS MEASUREMENT OFMarch 24, 1970 CONCENTRATION IN THE ATMOSPHERE OF THE SHORT-LIVED SOLIDDECAY PRODUCTS OF RADON 4 Sheets-Sheet 1 Filed March 25, 1968 a Q Q a Qa a N Qkl 9&5 .w E 5% 6 S .w .n k w k A R a L w WQQQ a; 9 www m I R illI k .MQAW QR Q x E? w 3% Q QSNQ y m GSN EN www March 24, 1970 c. LASSEUR3,502,876

APPARATUS FOR CONTINUOUS AND SIMULTANEOUS MEASUREMENT OF CONCENTRATIONIN THE ATMOSPHERE OF THE SHORT-LIVED SOLID DECAY PRODUCTS OF RADON 4SheetsSheet 2 Filed March 25, 1968 c. LASSEUR 3,502,876

MENT OF CONCENTRATION IN THE ATMOSPHERE OF THE SHORT-LIVED March 24,1970 APPARATUS FOR CONTINUOUS AND SIMULTANEOUS MEASURE SOLID DECAYPRODUCTS OF RADON 4 Sheets-Sheet 5 Filed March 25, 1968 WOC m) 4 VS QQKUm WM c. LASSEUR' March 24, 1970 APPARATUS FOR CONTINUOUS ANDSIMULTANEOUS MEASUREMENT OF CONCENTRATION IN THE ATMOSPHERE OF THESHORT-LIVED SOLID DECAY PRODUCTS OF RADON 4 Sheets-Sheet 4 Filed March25, 1968 qoE United States Patent 3,502,876 APPARATUS FOR CONTINUOUS ANDSIMUL- TANEOUS MEASUREMENT OF CONCENTRA- TION IN THE ATMOSPHERE OF THESHORT- LIVED SOLID DECAY PRODUCTS 0F RADON Claude Lasseur, Clamart,France, assiguor to Commissariat a lEnergie Atomique, Paris, FranceFiled Mar. 25, 1968, Ser. No. 715,920 Claims priority, applicatio6ng'rance, Apr. 13, 1967,

Int. 01. don N18 US. Cl. 250-83.6 6 Claims ABSTRACT OF THE DISCLOSUREThis invention is concerned with an apparatus for taking continuous andsimultaneous measurements of concentration in the air of the short-livedsolid decay products of radon and to deduce therefrom any disequilibriumwhich may exist.

The devices which are at present employed estimate either theconcentration of radon gas or the total alpha activity or the totalbeta-activity but no device gives the natural activity of eachradionuclide, namely: radon, radium A (RaA), radium B (RaB), radium C(RaC) or radium C (RaC).

It should also be noted that the majority of measurements of the naturalradioactive component of the atmosphere which is due to radon and itsshort-lived solid decay products are frequently carried out withnon-specialized materials whose main function is in fact the measurement of the artificial component of atmospheric conta-mi nation.

The present invention provides a remedy to these disadvantages andshortcomings of the prior art since it proposes an apparatus which isspecially designed to follow separately the evolution of eachshort-lived solid decay product of radon.

More specifically, said apparatus for the continuous measurement of theconcentration in the atmosphere of short-lived solid elements resultingfrom the decay of radon, radium A, radium B, radium C and radium C, bymeasuring the activity of each of said elements, is characterized inthat it comprises:

A filter strip,

A system for moving said strip,

A sampling tube traversed by said strip, said tube being open at one endand connected at the other end to a pumping device through theintermediary of an electrovalve,

An alpha-radiation detector and a beta-radiation detector placed inoppositely-facing relation, one detector being placed above the stripand the other detector being placed beneath said strip, both detectorsbeing placed at a predetermined distance from said sampling tube,

A cyclic control assembly for delivering on the one hand the pulse whichopens said electrovalve and defines the commencement of a sampling stageand on the other hand for initiating in particular the operation of thestripmoving system after a given sampling time,

Means for stopping said strip-moving system when that 3,502,876 PatentedMar. 24, 1970 portion of the strip which is present within said tubeduring the sample stage is located in front of the detectors,

An electronic circuit for collecting the pulses supplied by thealpha-radiation detector, comprising especially a two-channel selectorwhich is intended to effect the separation of the pulses correspondingto the alpha-particles emitted by radium A and radium C, and twointegrators each disposed in one of the output channels of said selectorand supplying a signal which is proportional in one case to the activityof the radium A and in the other case to the activity of the radium Cwhich are present on the strip,

An electronic circuit for collecting the pulses supplied by thebeta-radiation detector, comprising especially an integrator whichsupplies a signal proportional to the total activity of the beta-emitterelements radium B and radium C, and

A potentiometric assembly which is intended to subtract the signalrepresenting the activity of RaC deduced from the signal representingthe activity of RaC from the signal representing the total activity ofthe betaemitters and which therefore supplies a signal which isproportional to the activity of radium B.

Further properties and advantages of the present invention will becomeapparent from the description which now follows below, reference beingmade to the accompanying drawings in which one form of construction ofsaid apparatus is given by way of explanation but not in any limitingsense.

In these drawings:

FIG. 1 is the general diagram of decay of the daughter products ofradon,

FIG. 2 shows a family of curves showing the evolution on a stationaryfilter as a function of the sampling time t, in hour, of eachshort-lived decay product of radon and more especially of the ratio inwhich A(t) represents the activity of the product at the time t and Arepresents its activity at equilibrium on the filter. It should bepointed out that these curves are established in the case of equilibriumof the radioactive relationship in the atmosphere. Only the curverelating to RaA always has the same shape.

FIG. 3 is a diagram of the apparatus according to the invention withoutits electronic measuring assembly and, finally,

FIG. 4 is the electronic diagram of the measuring assembly.

The evolution of radioactive decay of radon in the atmosphere as well asits collection on a stationary filter call in the first place for thefollowing comments:

(1) There exist two theoretical extreme cases of evolution in theatmosphere, both of radon and its solid decay products: the radioactiveequilibrium in which all the decay products have the same activity whichis constant in time, and the state of equilibrium in which radon gas isalone present in the initial stage and not renewed. In this second case,each element accordingly tends towards a state of equilibrium with theradon gas.

In the case of a same number of atoms of RaA at the initial instant, itis shown that, at any given that t, the activity of RaA alwaysrepresents the same fraction of the activity of radon at the initialtime t irrespective of the state of evolution.

(2) A collection on a stationary filter or membrane dissociates thedecay products. In fact, as the gas passes through the filtering layer,only the solid elements are retained. The evolution of radioactive decayon the filter is therefore different from the evolution in the atmos- 3phere. However, the activity of each element deposited attains anequilibrium value.

It is demonstrated that, at any given time t, the activity of the decayproduct RaA on the filter always represents the same fraction of itsquilibrium activity (FIG. 2).

The two foregoing conclusions therefore make it possible to consider RaAas a reference element for the radon. For a same sampling time and atequilibrium between Rn and RaA, the activity of the RaA deposited on thefilter represents a constant fraction of the initial activity of theradon. The concentration of radon gas in the atmosphere can therefore beobtained without entailing measurement.

However, in order to follow the evolution of the decay process withaccuracy, the sampling time must be as short as possible. Said samplingtime must also be compatible with a deposit activity on the filter whichis sufficient to ensure good statistical counting precision over thefull range of measurement. The apparatus according to the invention isdesigned for a sampling time of three minutes.

The principle of the apparatus is also based on the correctspectrometric separation of the alpha emitters RaA and RaC. Theapplication of this method of measurement has become possible onlythrough the development of large-surface semiconductor detectors. Theuse of such detectors in an industrial apparatus is relatively simplewhereas any other known alpha spectrometer remains essentially alaboratory apparatus. The spectrometry is also improved by the use of amillipore filter membrane which minimizes the energy degradation whichis always present in an atmospheric sample on a filter. The junctiondelivers pulses having amplitudes which are proportional to the energyof the two emitters to be separated (RaA6 mev. and RaC'-7.68 mev.). Theseparation is carried out by means of a two-channel selector, theseparation threshold of which is adjustable in such a manner as tocompensate for any possible displacement of the peaks. Measurement ofthe concentration of RaC in the atmosphere also gives the RaCconcentration to within 0.04% inasmuch as the latter element decays witha yield of 99.96% to form RCa.

The measurement of RaB constitutes another essential point of theinvention. This element is a beta emitter as is also RaC. Any separationat the detection level is therefore impossible. However, the activity ofRaC is known from its alpha measurement after a correction of 0.04%. Anelectronic subtraction total beta-activity- RaC activity is thereforepossible after shaping and integration and results in the beta-activityof RaB alone.

Finally, in order to prevent any contamination of the junction, thesampling and measurement are carried out at two different points but,since RaA has a half-life of 3.05 minutes, the movement of translationfor sampling beneath the detectors must be rapid. A correction on thedecay of said element must be applied in order to take into account thetime of transfer and measurement.

The principle of the apparatus is therefore based on four essentialconcepts:

The theoretical considerations relating to the radioactive decay ofradon and its evolution in the atmosphere and on the stationary filter.

The use of a large-surface junction and of a millipore filter membranefor the purpose of obtaining a good spectrometric separation of thealpha-emitters.

The electronic measurement of RaB by subtraction from the totalbeta-activity of the activity of the RaC which is deduced from its alphameasurement.

The separation of the measurement sample followed by rapid displacementbeneath the detectors.

As can be seen from FIG. 3, the apparatus comprises a filter strip 1having a width of centimeters which is wound at one end onto a payoffreel 2 and at the other end onto a takeup reel 3 which is driven by amotor 4 fitted with an electromagnetic clutch unit 5.

The filter strip 1 passes through the lower portion of a sampling tube6. This latter has an extension in the form of a duct 7 of smallerdiameter which is connected to a pumping plant (not shown) and fittedwith a primary-input electrovalve 8 and a secondary-input electrovalve9. The sampling area is 8 cm. The moving armature 11 of an electromagnet10 is coupled to a punch 12 and an inner tube 13 which is slidablyfitted within the sampling tube 6. In the lowermost position, the innertube 13 rests on the membrane and maintains it against the bottomportion of the sampling tube 6. A timing cam 14 which is driven by amotor 15 produces action on a relay 16 which controls the electrovalves8 and 9 as well as the electromagnet 10. A photoresistive cell 17 whoseexciter lamp is shown at 18 also controls the electromagnet 10 by meansof a second relay 19. The excitation contact 20 of the electromagnetclutch unit is located at the top of the punch 12.

The detection unit comprises a semiconductor junction 21 placed abovethe filter membrane 1 and Geiger-Muller counter 22 which is placedopposite to the junction and beneath said membrane. The junction 21 hasa useful area of 9.5 cm. and carries out the spectrometric separation ofthe alpha-emitters (RaA and RaC) whereas the counter 22 which isprovided with an input window formed of mica (4 mg./cm. and having auseful area of 20 cm. detects only beta-radiations (alpha-radiationsbeing stopped by the membrane and by the mica window).

An anticoincidence Geiger-Muller tube 23 having a semi-annularcross-section surrounds the non-detecting surface of the counter 22.

The distance D between the axis of the inner tube 13 and the axis of thepunch 12 is the same as the distance from the center of the detectors tothe center of the photoresistive cell 17.

The electronic measuring unit is shown diagrammatically in FIG. 4. Thepulses delivered by the junction 21 are received in a discriminator 26after having passed through a preamplifier 24 and an amplifier 25. Theseparation of the pulses corresponding to the alpha-particles emitted byRaA and RaC is carried out by means of a two-channel selector 27. Theparallel output channels of the pulses due to RaA and to RaC areidentical and each comprise a shaping circuit (28 and 29), an integrator(30 and 31) and a measuring amplifier (32 and 33). Said amplifiers arecoupled respectively with the channels A and C of a recorder 34.

The information collected in channel A therefore gives the activity ofthe RaA which is present on the membrane. After calibration anddetermination of the quantity of air which has passed through saidmembrane during the sampling period, it is thus possible to determinethe concentration of RaA in the atmosphere. The recorder can even begraduated directly in concentrations. Moreover, since the measuredactivity in said channel represents as previously stated a constantfraction of the activity of the radon, it is also possible to deducefrom this measurement the concentration of radon gas in the atmosphere.

Similarly, the information collected in channel C makes it possible toobtain the concentration of RaC and consequently the concentration ofRaC to within 0.04%.

An additional measuring channel may be provided for the purpose ofrecording the total u-activity taken at the output of the amplifier 27after shaping and integration which is similar to the two other or.channels.

The measuring Geiger-Muller counter 22 and anti-coincidence counter 23deliver pulses which are received respectively by the preamplifiers 35and 36, then shaped in the circuits 37 and 38. Said pulses meet again atthe input of an anticoincidence circuit 39 which has the function ofreducing the background of the counting device by eliminating all pulsesother than those which are derived from the decay of the beta-emittersretained on the filter strip 1.

The anticoincidence circuit 39 is followed by a shaping circuit 40, anintegrator 41 and a measuring amplifier 42 whose output is coupled withthe channel T of the recorder 34. The information which serves to obtainthe total activity of the beta-emitters appears in said channel T.

Between the integrator 31 and the measuring amplifier 33, the channelfor the measurement of RaC is provided with a branch comprising a device43 for effecting electronically the correction of yield of 0.04% whichmakes it possible to obtain at its output pulses corresponding to theactivity of the RaC. These pulses are then shaped at 44, then integratedat 45. The integrator 45 is coupled on the one hand to the input of themeasuring amplifier 42 and on the other hand to the input of a measuringamplifier 46 which is in turn coupled to the output of the amplifier 42via a potentiometer 47, the movable contact of which is connected tochannel B of the recorder 34.

An arrangement of this kind carries out the electronic subtraction:

Total beta-activity.-Activity of RaC and the information collected inchannel B relates to the activity of RaB alone; said informationtherefore makes it possible to obtain its concentration in theatmosphere.

The recorder in accordance with the invention operates as follows:

The filter strip 1 which is placed directly in the air stream of thesampling tube 6 collects the short-lived solid decay products of radon,namely RaA, RaB, RaC and RaC.

The moving armature 11 of the electromagnet 10, the movable inner tube13- of the sampling tube 6 and the punch 12 are accordingly in thebottom position (the top position is shown in chain-dotted line in FIG.3). A perforation is made in the strip by said punch.

After three minutes of sampling, the timing cam 14 closes the generalcontact which energizes the control relay 16 of the electrovalves -8 and9 and of the electromagnet 10. The primary-input electrovalve 8 isaccordingly cut off and the secondary-input electrovalve 9 is energized,with the result that the filter strip 1 is no longer applied against thebottom portion of the sampling tube 6. At the same time, theelectromagnet 10 is energized, its moving armature 11 being thusattracted upwards and accompanied in its movement by the movable innertube 13 of the sampling tube as Well as by, the punch 12. The upwardmotion of said punch causes the excitation contact of theelectromagnetic clutch unit 5 to close at 20, thereby coupling thedriving motor 4 to the takeup reel 3 of the membrane strip.

The clutch unit 5 being energized, the punch 12 and the inner tube 13being in the top position, the sample then moves beneath the detectors.When the hole formed by the punch 12 in the filter strip reaches thelevel of the photo-resistive cell 17, said cell receives a luminous fiuxwhich it converts to electric current. This current then energizes therelay 19 which cuts off the supply of current to the electromagnet 10.The armature 11 is then released and drives downwards the movable innertube 13 of the sampling tube as well as the punch 12 which are thuslocated once again in their initial positions. A further perforation isthen made in the strip 1. The downward motion of the punch has theeffect of opening at 20 the excitation contact of the electromagneticclutch unit 5. The reels no longer rotate and the sample is thuspositioned exactly in front of the detectors 21 and 22.

On completion of the above-described transfer operations which arecarried out in less than twenty seconds, the four measurements arerecorded at a rate of one channel every four seconds. Thus, the transferand the recording are carried out in twenty seconds. Twenty secondsafter the initial pulse, the timing cam 14 delivers a second pulse whichrestores the relays 1-6 and 19 to their rest positions. Theprimary-input electrovalve 8 is then energized whilst thesecondary-input electrovalve 9 is cut off. The membrane is againmaintained applied against the lower portion of the sampling tube 6. Afurther sampling and a further measurement may then be carried out. I

It will be noted that all the electric devices with the exception of theelectrovalve 8 are in the rest pisition while the counting and samplingoperations are being performed. The control of the electromagneticclutch unit 5 by means of the punch 12 is a safety feature. The filterstrip can only be run-01f if the sleeve 13, the punch 12 and thearmature 11 of the electromagnet are in the top position. Any possibletearing of the membrane is thus avoided.

Starting from the first cycle, the photoresistive cell 17 iscontinuously energized by the luminous flux. A timedelay circuit (whichis not shown in the figures) renders the cell inoperative for a periodof time which is sufficient to permit the paper to move forward by a fewmillimeters. This solution is preferred to a cyclic interruption of theexciter lamp 18, inasmuch as repeated interruption would in fact damagethe lamp in a very short time.

The separate measurement of the decay products of radon which can becarried out by means of the apparatus in accordance with the inventionis of considerable practical interest. In point of fact, it is assumedin the majority of devices for measuring contamination that radon andits short-lived daughter elements are in a state of radioactiveequilibrium. However, the concentrations of radon undergo substantialday-time or night-time variations (a factor of Consequently, theequilibrium is upset and it becomes necessary to follow the evolution ofeach daughter element separately. This distinction oifers a substantialadvantage in a certain number of cases.

The most immediate application of the apparatus is in mines in which theconcentrations are of the order of (or higher than) the maximumpermissible concentration of radon. This latter undergoes substantialfluctuations (factor of 100 to 1,000) over very short time intervalsduring mining of ore by blasting since the radon which is trapped withinthe rock is released instantaneously. The constant evolution of the gasand of its solid decay products produces substantial variations inaccumulated doses at the level of the bronchi in mine workers and it isuseful to know at each instant the concentration of each decay productin the atmosphere.

Moreover, radon is widely employed in meteorology as radioactive tracerfor the study and measurement of various parameters.

A knowledge of the evolution of each element must therefore permit muchfiner measurements than those 'which are based on the total alpha orbeta activity, especially during sharp variations in diffusionparameters (wind, temperature gradient and so forth).

Finally, the separate measurements of the decay products of radon is ofinterest in many other cases, of which a few examples are given belowwithout any intended limitation:

(a) Measurement of the natural radioactivity in building premises andcomparison with the surrounding atmosphere.

(b) Measurement of the natural radioactivity in special premises such asventilated premises which are supplied with either filtered orunfiltered air, halls of reactor buildings and so forth.

(c) Measurement of the artificial radioactive contamination, alpha(PuU). This measurement is possible by displacement of the lower andupper thresholds of the RaA channel. There is obtained a channel ofvariable width which isolates the peak of the emitter to be selected.

(d) Comparison between the measurements of natural radioactivity givenby contamination devices which are not suited to this type ofmeasurement and those of the apparatus under consideration.

It will be apparent that the present invention has been described in theforegoing by way of explanation but not in any sense by way oflimitation and that any detail modifications may be contemplated withoutthereby departing from the scope of the invention.

In the apparatus hereinabove described, sampling and measurement areperformed in three minutes and the transfer operations in twenty secondsbut it is understood that any other cycle may be employed, in which caseit is merely necessary to make use of a different timing cam.

What I claim is:

1. An apparatus for the continuous measurement of the concentration inthe atmosphere of short-lived solid elements resulting from the decay ofradon, namely, radium A, radium B, radium 'C and radium C, by measuringthe activity of each of said elements, and comprising:

a filter strip,

a system for moving said strip,

a sampling tube traversed by said strip, said tube being open at one endand connected at the other end to a pumping device through theintermediary of an electrovalve,

an alpha-radiation detector and a beta-radiation detector placed inoppositely-facing relation, one detector being placed above the stripand the other detector being placed beneath said strip, both detectorsbeing placed at a predetermined distance from said sampling tube,

a cyclic control assembly for delivering on the one hand the pulse whichopens said electrovalve and defines the commencement of a sampling stageand on the other hand for initiating in particular the operation of thestrip-moving system after a given sampling time,

means for stopping said strip-moving system when that portion of thestrip which is present within said tube during the sampling stage islocated in front of the detectors,

an electronic circuit for collecting the pulses supplied by thealpha-radiation detector, comprising espe cially a two-channel selectorwhich is intended to effect the separation of the pulses correspondingto the alpha-particles emitted by the radium A and radium C, and twointegrators each disposed in one of the output channels of said selectorand supplying a signal which is proportional in one case to the activityof the radium A and in the other case to the activity of the radium Cwhich are present on the strip,

an electronic circuit for collecting the pulses supplied by thebeta-radiation detector, comprising especially an integrator whichsupplies a signal proportional to the total activity of the beta-emitterelements radium B and radium C,

and a potentiometric assembly Which is intended to subtract the signalrepresenting the activity of RaC deduced from the signal representingthe activity of RaC from the signal representing the total activity ofthe beta-emitters and which therefore supplies a signal which isproportional to the activity of radium B.

2. An apparatus in accordance with claim 1, characterized in that thesystem for moving the filter strip comprises:

two reels onto which said strip is wound at both ends,

and a motor for driving one of said reels in rotation and fitted with anelectromagnetic clutch unit.

3. An apparatus in accordance with claim 1, characterized in that thecyclic control assembly comprises:

a timing cam,

an electromagnet whose armature controls the excitation contact of theelectromagnetic clutch unit,

and an electric relay controlled by said cam and adapted to initiate onthe one hand the opening and closure of said electrovalve and on theother hand the energization of said electromagnet.

4. An apparatus in accordance with claim 1, characterized in that saidmeans for stopping the strip-moving system comprise:

a punch placed on the path of said strip at a distance D from thesampling tube, said punch being coupled to the armature of saidelectromagnet so as to produce a perforation in the strip when saidelectromagnet is not energized,

a photosensitive cell and a light source for exciting said cell whichare disposed in oppositely facing relation above and beneath the stripat a distance D from the radiation detectors, said photosensitive cellbeing connected to a relay which cuts ofi? the supply of current to theelectromagnet when the cell is energized, thus opening the contact whichserves to energize the electromagnet clutch unit of the motor.

5. An apparatus in accordance with claim 1, characterized in that thealpha-radiation detector is a semiconductor junction.

6. An apparatus in accordance with claim 2, characterized in that thebeta-radiation detector is a Geiger- Muller tube having an input windowof sufiicient thickness to stop the alpha-particles of radium A andradium C.

References Cited UNITED STATES PATENTS 2,892,091 6/1959 SaWle 25043.5 X2,972,678 2/1961 Anton 25083.6 X 3,092,723 6/1963 Payne et al. 250106 X3,109,096 10/1963 Spaa 250-83.6 X

ARCHIE R. BORCHELT, Primary Examiner D. L. WILLIS, Assistant ExaminerUS. Cl. X.R. 250-106

